Microelectronic devices are fabricated on semiconductor (e.g. silicon) wafers using a variety of techniques, e.g. including deposition techniques (CVD, PECVD, PVD, etc.) and removal techniques (e.g. chemical etching, CMP, etc.). Semiconductor wafers may be further treated in ways that alter their mass, e.g. by cleaning, ion implantation, lithography and the like.
Depending on the device being manufactured, each semiconductor wafer may be passed sequentially through hundreds of different processing steps to build up and/or to remove the layers and materials necessary for its ultimate operation. In effect, each semiconductor wafer is passed down a production line. The nature of semiconductor manufacturing means that certain processing steps or sequences of steps in the production flow may be repeated in a similar or identical fashion. For example, this may be to build up similar layers of metal conductors to interconnect different parts of the active circuitry.
To ensure consistency and interoperability of semiconductor equipment used in different factories, standards are adopted throughout the majority of the semiconductor manufacturing industry. For example, standards developed by Semiconductor Equipment and Materials International (SEMI) have a high degree of market uptake. One example of standardisation is the size and shape of the semiconductor (silicon) wafers: typically for volume production they are discs having a diameter of 300 mm. However, some semiconductor (silicon) wafers (typically used in older factories) are discs having a diameter of 200 mm.
The cost and complexity of the processing steps required to produce a completed silicon wafer together with the time that it takes to reach the end of the production line where its operation can be properly assessed has led to a desire to monitor the operation of the equipment on the production line and the quality of the wafers being processed throughout processing, so that confidence in the performance and yield of the final wafers may be assured.
Wafer treatment techniques typically cause a change in mass at or on the surface of the semiconductor wafer. The configuration of the changes to the surface are often vital to the functioning of the device, so it is desirable for quality control purposes to assess wafers during production in order to determine whether they have the correct configuration.
Specialist metrology tools may be used within the production flow so that monitoring is conducted soon after the relevant process of interest and usually before any subsequent processing, i.e. between processing steps.
Measuring the change in mass of a wafer either side of a processing step is an attractive method for implementing product wafer metrology. It is relatively low cost, high speed and can accommodate different wafer circuitry patterns automatically. In addition, it can often provide results of higher accuracy than alternative techniques. For example, on many typical materials, thicknesses of material layers can be resolved down to an atomic scale. The wafer in question is weighed before and after the processing step of interest. The change in mass is correlated to the performance of the production equipment and/or the desired properties of the wafer.
Processing steps carried out on semiconductor wafers can cause very small changes in the mass of the semiconductor wafer, which it may be desirable to measure with a high accuracy/sensitivity. For example, removing a small amount of material from the surface of the semiconductor wafer may reduce the mass of the semiconductor wafer by a few milligrams and it may be desirable to measure this change with an accuracy of the order of ±100 μg or better. Conventional semiconductor wafer metrology methods and devices that rely on measuring the mass or the change in mass of a semiconductor wafer generally only have an accuracy of the order of ±100 μg under normal operating conditions. This accuracy may be insufficient for very high sensitivity measurements of mass changes, or for measurements of very small mass changes, e.g. a mass change due to ion implantation. In addition, as smaller and lighter semiconductor wafers become commercially available the mass changes caused by processing of the semiconductor wafers will decrease, and higher accuracy measurements than can be achieved with the conventional methods and devices will be required to monitor the processing and/or to check the subsequent semiconductor wafers.
Therefore, there exists a problem of measuring the mass or the change in mass of a semiconductor wafer with greater accuracy.